European patent No. 230.021 describes a process in which glucose is fermented continuously to lactate, after which lactic acid is extracted from the solution by means of electrodialysis, where pH in the fermentor is controlled by removing the lactic acid at the same rate as the rate at which it is formed, the contents of the fermentor being recirculated over the electrodialysis unit. Yeast extract and inorganic salts are used as nutrients. A disadvantage of this system is that bacteria in the fermentor liquid are known to adsorb to the electrodialysis membranes, causing the electrical resistance in the electrodialysis unit to increase, which results in a substantially increased power consumption for the electrodialysis process.
Boyaval et al. (Biotechnology Letters Vol. 9, No. 3, 207-212, 1987) describe a bioreactor for lactic acid fermentation using a three-stage fermentation process that includes the production of biomass and lactic acid in the first stage, separation and concentration of the cells by ultrafiltration in the second stage, and lactate concentration and purification by electrodialysis in the third stage. It is reported, however, that this system exhibits the disadvantage of clogging of the ultrafiltration membranes, resulting in drastic restriction of permeate flow.
U.S. Pat. No. 4,110,175 also describes a general method for electrolytic purification of organic acids, including lactic acid. An improved version of this method is described in U.S. Pat. No. 5,002,881, in which lactic acid is formed as ammonium lactate through fermentation of a glucose-containing medium, which makes it possible to use ultrafiltration to separate the ammonium lactate from the fermentation liquid, as the retentate from the ultrafilter is returned to the fermentor. In this way there is no adsorption of bacteria to the membranes in the subsequent electrodialysis processes, and power consumption is therefore lower. The microorganism used in the patent is Bacillus coagulans, which has the property of not needing any special nutrient medium containing yeast extract or corn steep liquor, which are otherwise known to be necessary to maintain lactic acid fermentation when lactic acid bacteria are used. Prior to electrodialysis, the fermentor liquid is concentrated by means of reverse osmosis (RO), and the concentrated liquid is subsequently treated in an electrodialysis unit in which lactic acid is formed from ammonium lactate by means of bipolar membranes in a single operation. In this operation ammonium hydroxide is formed at the same time and can be returned to the fermenter as a medium for neutralisation of lactic acid. in this process, however, amino acids are used as a nutrient for the fermenting bacteria, which results in the disadvantage of relatively high costs. A further disadvantage is that RO used for concentration will result in non-converted organic matter (residual glucose and amino acids) being included in the electrodialysis treatment with bipolar membranes, where they contribute to reducing the process efficiency. Also, the resulting product might not be heat-stable due to the presence of residual sugars in the lactic acid.
The formation of amino acids from whey proteins and the use of whey protein as a nutrient in the fermentation of lactose in whey is described in U.S. Pat. No. 4,698,303. However, U.S. Pat. No. 4,698,303 has the disadvantage of requiring an independent hydrolysis for the production of amino acids from whey protein, the hydrolysis being carried out as a separate acidic enzymatic process, after which the hydrolysed product is fed to the membrane fermenter as a nutrient.
U.S. Pat. No. 5,503,750 discloses a method for the production and recovery of lactic acid using a combination of ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO).
WO 96/41021 discloses a method for recovering and purifying organic acids, e.g. lactic acid, using electrodialysis, where nanofiltration and contact with a chelating agent are employed to reduce fouling of ion-selective membranes used in electrodialysis.
EP 0393818-A discloses a method for purifying lactic acid using an electrodialysis step, a bipolar electrodialysis step, treatment with a strong cationic ion exchange resin and treatment with a weak anionic ion exchange resin.